1. Field of the Invention
This invention relates to apparatus for the detection of seismic waves using geophones. In one of its more particular aspects this invention relates to a device for positioning geophones in shallow holes.
2. Introduction
In the search for petroleum and other valuable resources, it has become the practice to transmit a seismic signal into the earth from one or more source points near the surface of the earth. The reflected and/or refracted energy returning from within the earth to a receiver location is detected by a geophone or other seismometer and raw seismic data are recorded. The raw seismic data are mathematically processed and then interpreted to provide an indication of the structure of the underlying strata.
Geophones that detect the reflected seismic signals are generally electromagnetic devices that contain at least one coil representing the electric circuit and at least one magnet which cooperates with the coil. Either the coil or the magnet is rigidly connected to a frame, while the other is elastically suspended from the frame. In response to earth motion, all of the fixed elements of the geophone move with the frame, but the suspended element, under the influence of inertia, tends to be delayed with respect to the unsuspended element, creating relative movement between the coil and magnet. This movement induces an electrical current in the coil which comprises the output signal of the geophone. In any of these geophones, there exists a primary axis, such as a horizontal axis or a vertical axis, along which the suspended element is free to reciprocate, thereby giving rise to the electrical output signal.
Geophones have been used to detect different types of generated seismic signals during seismic exploration of earth strata. The most common type is the so-called compressional (P) wave in which the earth particle motion is in the same direction as the wave propagation. Compressional waves are also sometimes called longitudinal waves. Another type is the shear wave in which the earth particle motion is normal to the direction of wave propagation. Shear waves in which the particle motion is oriented normal to the incident plane are called horizontal shear (SH) waves and shear waves in which the particle motion is oriented within the incident plane are called vertical shear (SV) waves.
Compressional waves are the signals most commonly used for seismic exploration. Compressional waves may be generated in numerous ways, such as the detonation of an explosive, the dropping of weights or the use of mechanical vibrators, and compressional waves are usually detected by so-called vertical geophones designed and manufactured with a vertical primary axis for detecting vertical earth movement. Recently, however, there has been an increased interest in the use of shear waves in seismic exploration. Cherry U.S. Pat. No. 3,286,783, Water U.S. Pat. No. 3,202,164, Barr, Jr. U.S. Pat. No. 3,736,556, and Turpening U.S. Pat. No. 4,059,820 disclose various apparatus and methods for the generation, detection, and use of shear waves in seismic exploration. Horizontal shear waves have generally been detected using horizontal geophones.
It has been proposed to use a combination of a vertical geophone and one or more horizontal geophones mounted on a common baseplate, known as a three-component geophone. Such combination is disclosed in Goff U.S. Pat. No. 4,300,220.
The positioning of geophones on the surface of the earth presents no particular problems, since it is a relatively simple task to adjust the level and alignment of surface employed geophones. However, the use of geophones in shallow holes, that is, holes of up to about 4 to 5 feet in depth, requires that the leveling and aligning be accomplished at the same time the geophone is placed in the hole, since subsequent repositioning is cumbersome and time consuming.